1. Field of the Invention
The present invention relates to a technique of determining whether or not an abnormality occurs to an oil temperature sensor used for detecting the temperature of a transmission oil of an automatic transmission for a vehicle.
2. Description of the Background Art
An automatic transmission for a vehicle is generally controlled by means of a transmission oil supplied from an oil hydraulic control circuit. The transmission oil is a fluid whose characteristic such as viscosity varies according to the temperature. It is therefore desirable to keep the oil temperature at an appropriate temperature in order to achieve a stable transmission feeling. If an abnormality occurs to an oil temperature sensor used for detecting the temperature of the transmission oil supplied to the automatic transmission, the transmission performance and the durability could be deteriorated. Thus, whether or not such an abnormality occurs should be determined as early as possible. Japanese Patent Laying-Open No. 2004-340172 for example discloses how to determine whether or not an oil temperature sensor is abnormal.
An oil temperature determination apparatus disclosed in Japanese Patent Laying-Open No. 2004-340172 detects the transmission response time of an automatic transmission, and calculates an estimate value of the transmission oil temperature based on the detected transmission response time. In the case where a temperature difference between the oil temperature detected by the oil temperature sensor and the estimate value of the oil temperature is equal to or larger than a predetermined value, the oil temperature determination apparatus determines that the oil temperature sensor is abnormal.
Further, Japanese Patent Laying-Open Nos. 08-303564 and 05-272622 each disclose how to estimate the temperature of the oil for an automatic transmission.
The oil temperature estimation apparatus disclosed in Japanese Patent Laying-Open No. 08-303564 estimates the amount of heat generated from the automatic transmission based on operating conditions of the automatic transmission, estimates the amount of heat dissipated into the atmosphere based on the vehicle speed and the ambient temperature of the automatic transmission, and estimates the temperature of the oil for the automatic transmission based on the estimated amount of the generated heat and the estimated amount of the dissipated heat.
The oil temperature estimation apparatus disclosed in Japanese Patent Laying-Open No. 05-272622 calculates the capacity coefficient of a torque converter from the engine speed and the engine output torque, calculates the speed ratio of the torque converter from the engine speed and the turbine speed, and estimates the temperature of the oil for the automatic transmission based on the capacity coefficient and the speed ratio of the torque converter.
While the oil temperature determination apparatus disclosed in Japanese Patent Laying-Open No. 2004-340172 calculates the estimate value of the oil temperature based on the transmission response time, the relation between the transmission response time and the temperature of the transmission oil varies depending on individual transmission units. In order to ensure the accuracy in estimation of the oil temperature, it is necessary to adapt in advance the relation between the transmission response time and the temperature of the transmission oil to each transmission unit. In this case, enormous work time is required for the adaptation.
As for the oil temperature determination apparatus disclosed in Japanese Patent Laying-Open No. 08-303564, the amount of heat dissipated into the atmosphere is estimated based on the vehicle speed and the ambient temperature of the automatic transmission. The relation between the transmission response time and the temperature of the transmission oil, as well as the relation between (i) the vehicle speed and the ambient temperature of the automatic transmission and (ii) the amount of heat dissipated into the atmosphere vary depending on individual transmission units and depending on individual vehicles where respective transmissions are mounted. In order to ensure the accuracy in estimation of the oil temperature, it is therefore necessary to adapt the relation between the transmission response time and the temperature of the transmission oil, as well as the relation between (i) the vehicle speed and the ambient temperature of the automatic transmission and (ii) the amount of heat dissipated into the atmosphere, to each transmission unit and each vehicle on which the transmission is mounted, which requires enormous work time for adaptation.
Further, the oil temperature estimation apparatus disclosed in Japanese Patent Laying-Open No. 05-272622 does not consider the amount of dissipated heat in estimating the temperature of oil for the automatic transmission. Therefore, in order to ensure the accuracy in estimation of the oil temperature, further improvements are necessary.